jet stream

The jet stream is a comparatively narrow current of air that moves around the northern and
southern hemispheres of the Earth in wavelike patterns. It can be likened
to a river of wind moving at high speed. The jet stream varies from about
100 to 400 miles (161 to 644 km) wide and 1 to 3 miles (1.6 to 4.8 km) thick.
Its strongest winds are generally encountered at about 30,000 feet (9,144
m) in altitude – in the troposphere.
Jet-stream winds usually have a speed of 150 to 300 miles per hour (241
to 482 km/h), but speeds up to 450 mph (724 km/h) have been recorded. Its
general motion is from west to east. However, there also is a tropical easterly
jet stream, which occurs during summer. This jet stream originates in the
upper troposphere near Burma and extends to the west of Africa, some 6,000
miles (10,000 km) to the west. The most extreme differences in temperatures
occur where the stream is the narrowest.

The jet stream shifts position frequently and actually migrates with the
seasons. Sometimes two streams flow across the United States, one along
the northern border and other well toward the south. The cruising range
of aircraft flying downwind within a jet stream is greatly increased. Pilots
anticipating high-altitude or long-range flights attempt to discover the
location of the stream and use it to their advantage.

For several decades, meteorologists have studied jet streams and how they
affect the movements of air masses. While the exact relationship is still
unknown, meteorologists agree that jet streams may act as a barrier between
cold air in the north and warm air in the south. During their meandering,
the streams appear to allow some cold air to flow southward and warm air
to flow northward. These flows undoubtedly affect the formation of cold
and warm air masses.